This invention generally relates to wireless code division multiple access communication systems. In particular, the invention relates to data detection of communications having non-uniform spreading factors in such systems.
In code division multiple access (CDMA) communication systems, multiple communications may be simultaneously sent over a shared frequency spectrum. Each communication is distinguished by the code used to transmit the communication. Data symbols of a communication are spread using chips of the code. The number of chips used to transmit a particular symbol is referred to as the spreading factor. One common spreading factor is sixteen (16), where sixteen chips are used to transmit one symbol. Typical spreading factors (SF) are 16, 8, 4, 2 and 1 in TDD/CDMA communication systems.
In some CDMA communication systems to better utilize the shared spectrum, the spectrum is time divided into frames having a predetermined number of time slots, such as fifteen time slots. This type of system is referred to as a hybrid CDMA/time division multiple access (TDMA) communication system. One such system, which restricts uplink communications and downlink communications to particular time slots, is a time division duplex communication (TDD) system.
One approach to receive the multiple communications transmitted within the shared spectrum is joint detection. In joint detection, the data from the multiple communications is determined together. The joint detector uses the, known or determined, codes of the multiple communications and estimates the data of the multiple communications as soft symbols. Some typical implementations for joint detectors use zero forcing block linear equalizers (ZF-BLE), Cholesky or approximate Cholesky decomposition or fast Fourier transforms.
Many of these implementations are designed for all the communications to have the same spreading factor. Simultaneously handling communications having differing spreading factors is a problem for such systems.
Additionally, it may be desirable to use different spreading factors within a same communication. Some symbols of a communication may be sent at a higher quality than other bits within the same communication. To illustrate, some TDD/CDMA systems transmit format combination indicator (TFCI) bits along with other data of the communication burst. It is desirable to transmit these TFCI bits at a higher quality than the payload data. One approach to increase the quality of certain bits and not others is to use different spreading factors. To illustrate, the TFCI bits may be sent with a SF of 16 and the other bits may be sent with a SF of 8, 4, 2 or 1.
Accordingly, it is desirable to be able to handle differing spreading factors in joint detection.